I decided to put an op-amp "wrapper" around the standard BMP clipper stages, and add my favourite tone control for real versatility. As drawn, it sounds really good, but has more top end than a normal BMP because there's no input loading effect. The capacitor in the feedback around the first op-amp stage could be increased in value (I have 47 nF there at the moment), to give a roll off similar to that which the original transistor input stage would cause, but other players like the brighter top end, so I've left the diagram as originally drawn (on a napkin, in a pub!)

This is a "halfway" design - I was going to add electronic switching, and some other modes of operation to the "full" version. You'll see that it can be switched to "Jumbo Tonebender" mode in this version.

In the next version, there will be "Stock Old", "Stock New", "Russian", "Creamy", "Jumbo", "Swollen" and may be other modes too. These will be selected by electronic switching. It also occurred to me that it might be worth allocating two sounds to two foot switches, so that it was easy to switch between a low gain "Russian" mode and a screaming "Creamy" mode.... The possibilities are endless, and I know I'm going to end up building loads of these with preset resistors inside and DIP switches to select sounds, and no external controls except the bypass footswitch!

Here's the circuit schematic as it stands today, and this is a really worthwhile version:

BMP21.png

Enjoy!

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mictester wrote:I decided to put an op-amp "wrapper" around the standard BMP clipper stages, and add my favourite tone control for real versatility. As drawn, it sounds really good, but has more top end than a normal BMP because there's no input loading effect ...

excellent design. i love BMP circuits!

one question:is that some of the straight signal going into pin 5 of the 2nd op-amp stage? if so, is that to retain some pick attack? ( sorry, two questions )

mictester wrote:I decided to put an op-amp "wrapper" around the standard BMP clipper stages, and add my favourite tone control for real versatility. As drawn, it sounds really good, but has more top end than a normal BMP because there's no input loading effect ...

excellent design. i love BMP circuits!

one question:is that some of the straight signal going into pin 5 of the 2nd op-amp stage? if so, is that to retain some pick attack? ( sorry, two questions )

No. Pin 5 just sees bias voltage (½ of the supply voltage), which is filtered by an electrolytic capacitor to ground. Biasing the two halves of the op-amp package like this is quite common.

I installed a switch to remove from the circuit the 2 first clipping diodes a la tonebender jumbo, but something seems weird . In this mode the sound is like if I mixed "dry" signal with the fuzz one. I think I saw semone complaining about that too but I'm not really sure.

MicMicMan wrote:given the way it's biased and the impedance of that tonestack, maybe you have some part of the signal circulating through the biasing part of the circuit ?

No. There's a 10µF capacitor from the junction of the biasing resistors to ground to ensure that there's no audio on the bias (10µF looks like an AC short in this kind of circuit).

I built another of these last week (an order from a local guitarist), and included CMOS switching. I'll post the amended diagram including the CMOS. The switching was entirely silent, and there was no residual "break through" problem. I was more than happy with it!

DrNomis wrote:I was thinking of building myself a Big Muff Pi,but making some mods to it like replacing the original tone control circuit with a Marshall tonestack....

Go ahead and try it, but you'll find that you might have to reconsider the impedances around the tone stack. I find that the two band "James" circuit gives a huge range of tones, including the traditional "scooped" tone.

because it's directly linked to the output level of the effectnot using a divider as a volume at the end of the effect, but rather playing with the gain of the last "buffer" stage allows you to have a constant (and very low) output impedance

MicMicMan wrote:because it's directly linked to the output level of the effectnot using a divider as a volume at the end of the effect, but rather playing with the gain of the last "buffer" stage allows you to have a constant (and very low) output impedance

Thus the question remains, why is a gain control called "level"? (while the only reason it will suggest level is because that tonestage gives such an attenuation that with a make up gain of 2 it will still suggest being turned all the way down)

Other than that, what's the reason for the series-connection of those 1n and 10n caps?